The invention relates to a device for stabilizing a vehicle having an engine and an automatic transmission for driving drive wheels of a first axle, the transmission having a free-wheeling position for interrupting the force flux between the drive wheels and the engine, and having rotation-sensing wheel sensors on the drive wheels and on wheels of a second axle of the vehicle. The invention also relates to a method for this purpose.
Such a device or such a method are known, for example, from German Laid-Open Patent Application DE 199 12 332 A1. The device shifts the transmission automatically into the free-wheeling position if a skidding process has been detected using the rotational-speed wheel sensors. Here, a skidding value is determined using the wheel sensors and using a yaw sensor in a way which is not explained in more detail. When a limiting value is exceeded, the transmission is shifted into the freewheeling or neutral position, as a result of which the force flux in the drive train is interrupted. At any rate, the driver no longer needs to shift the transmission himself into the “neutral” position in order to return the vehicle to a stable state.
Other devices from the prior art interact with clutches which mechanically decouple the engine from the drive wheels in drive concepts with manually shifted transmissions. For example, according to German Laid-Open Patent Application DE 35 28 389 A1, there is provision that when a predetermined difference in rotational speed between the driven and non-driven wheels of the vehicle is exceeded, the clutch is disengaged and the clutch is engaged again when said difference drops below this value. This device has the purpose of regulating the drive slip of drive wheels so that it is not necessary to brake the slipping drive wheels. The objective is therefore to bring about the best case of propulsion possible.
An antilock brake regulator according to German Laid-Open Patent Application DT 2 251 548 has a different purpose. This antilock brake regulator automatically disengages the clutch if the vehicle goes into an antilock braking state or skidding state owing to an emergency braking operation, in order to prevent the engine stalling. In this state, the antilock brake regulator is active, that is to say it ensures that the brakes are alternately loaded and relieved of loading so that they do not lock.
The device according to German Laid-Open Patent Application DT 1 953 499 in which, when an antilock brake device is active per se, an antilock brake state of driven rear wheels of a vehicle is sensed, and in this case a vehicle clutch is decoupled in order to prevent the engine stalling, also has a similar objective. Between two antilock braking states, the drive wheels are usually made to rotate again so that the engine does not stall. However, this is not possible when there is too little friction between the ground and drive wheels. In this situation, the device according to DT 1 953 499 ensures that the clutch is disengaged.
In a device in German Laid-Open Patent Application DE 196 35 809 A1, the device according to DE 35 28 389 A1 is developed for the operating case of overrun conditions in which the braking effect of the engine comes into play. If this braking effect becomes too large, a difference in rotational speed between the driven and the non-driven wheels exceeds a limiting value, a friction clutch is disengaged.
However, in a vehicle with an automatic transmission, the locking of the drive wheels can, due to the principle, not lead to the engine stalling. Nevertheless, there is the problem, as in the device according to DE 199 12 332 A1, that the transmission is not shifted, automatically or manually by the driver, into the free-wheeling position until after a skidding situation occurs. The vehicle is then already no longer moving in a stable fashion.
The object of the present invention is therefore to develop a device and a method of the type mentioned at the beginning to the effect that the vehicle is as far as possible prevented from skidding.
This object is achieved with the device by virtue of the fact that it has gear shift means for shifting the transmission into the free-wheeling position when it is determined, using at least one of the rotation-sensing wheel sensors, that at least one wheel of the second axle locks or is expected to lock. Corresponding measures are provided in the method.
A locking situation occurs relatively late at the wheels driven by the engine owing to the driving effect of the engine. In comparison, the wheels of the second axle, which are usually not driven, tend to lock comparatively early. The presence of such locking, or the expectation of locking, is sensed according to the invention. The drive effect of the engine on the drive wheels which prevents locking at said drive wheels is interrupted by the shifting of the transmission into the free-wheeling position so that the vehicle is kept in a state of stable movement or, if appropriate, goes into this state again. The vehicle continues to be steerable. Intervention by the driver, for example by shifting the transmission manually into the free-wheeling or neutral position, is not necessary.
The drive wheels are advantageously still not locked before the shifting into the free-wheeling position owing to the driving effect of the engine.
The device according to the invention expediently actuates the engine, or an engine controller controlling it, in order to decrease the engine power, which brings about a further improvement in the driving stability.
The transmission is advantageously shifted by the device according to the invention into the free-wheeling position only below a predetermined speed. A very low selection can be made for the speed. It is possible, for example, for the vehicle to be just short of complete standstill.
Particularly in this speed range, an antilock brake system of the vehicle is inactive, with the result that it becomes possible for the vehicle to stop completely.
Although this operating state can in theory be determined automatically by the device according to the invention, for example in that the current speed is interrogated by a speed sensor or in that the rotational speed signals of the wheel sensors signal a correspondingly low rotational speed of the wheels. However, the antilock brake system preferably transmits a control signal to the device according to the invention. The antilock brake system signals, as it were, its “inactive state” so that the device according to the invention does not need to determine said state itself.
In the two advantageous refinements of the invention below, the device detects a predetermined braking deceleration of the vehicle on the basis of which it shifts the transmission into the free-wheeling position. The device can determine a deceleration value by reference to speed signals of a speed sensor and/or can determine corresponding reductions in the rotational speed at the wheels of the first and/or the second axle, which are determined by the rotation-sensing wheel sensors.
The transmission is preferably already shifted into the free-wheeling position when one or both wheels of the second axle lock, but this is not yet the case for the wheels of the drive axle.
A difference in rotational speed between in each case at least one wheel of the first axle and one wheel of the second axle is preferably evaluated as a further parameter for shifting the transmission into the free-wheeling position.
The concept according to the invention is expediently applied in a vehicle in which the wheels of the second axle are not driven by the engine. However, in principle it would also be possible for the wheels of the second axle also to be driven. The concept according to the invention can advantageously be applied, for example, even in vehicles in which the engine power is distributed non-uniformly so that, for example, the larger portion of the engine power is applied to the first axle and the smaller portion of the engine power is applied to the second axle. As a result, the wheels of the second axle tend to lock earlier since they have lower engine power.
There is expediently provision for the gear shift apparatus to shift the transmission back into a drive position above a predetermined speed.
The gear shift apparatus advantageously shifts the transmission out of the free-wheeling position and into a drive position after a predetermined time and/or when the wheel of the second axle no longer locks or is no longer expected to lock.
This drive position according to the above-mentioned measures is preferably that position into which the transmission was put before the shifting into the free-wheeling position. However, in principle it would also be possible for the transmission to go into another driving position, for example a lower driving setting corresponding to a lower speed of the vehicle, which is brought about by braking.
The device according to the invention can optionally be embodied using hardware and/or software. It may be a separate device or form a component of an antilock brake system, of a transmission controller or of a driving stability controller. It is also possible that parts of the functions of the device according to the invention are respectively implemented at a plurality of the above-mentioned systems.
The invention is explained below in more detail using an exemplary embodiment and with reference to the figure which shows a vehicle with the device according to the invention, with which the method according to the invention can be executed.